「大人 (おとな)」 means "adult." This word refers to a grown-up person, someone who is no longer a child. You use it when talking about adults in general, or when you contrast them with children.

成(せい)長(ちょう)した人(ひと)を指(さ)します。子(こ)供(ども)と大(おと)人(な)がいます。